An anode oxidation treatment in which an anode oxide film is formed on a surface of a member composed of aluminum or an aluminum alloy as a base material (aluminum base material), thereby imparting plasma resistance, corrosion gas resistance, and the like to the base material has hitherto been widely adopted.
For example, vacuum chambers used for a plasma treatment apparatus of semiconductor manufacturing equipment, and various members to be provided in the inside of the vacuum chamber, such as electrodes, are usually formed using an aluminum alloy. However, when the aluminum alloy is used in a pure state, plasma resistance, corrosion gas resistance, and the like cannot be kept, and therefore, a treatment for imparting plasma resistance, corrosion gas resistance, and the like has been taken by applying an anode oxidation treatment on the surface of the member formed of the aluminum alloy to form an anode oxide film thereon.
Though the anode oxide film is formed with a different thickness according to an application thereof, in order to carry out the anode oxidation treatment, a direct-current power source is frequently used. In the case where the anode oxidation treatment is conducted with a constant current, the voltage increases with an increase of the thickness to produce a high voltage, and the aluminum base material is dissolved, and therefore, an anode oxidation-treated aluminum base material having a good thickness cannot be obtained. Though a relation between the thickness and the voltage and a voltage at which the aluminum base material is dissolved vary depending upon the treatment condition, the limit of the thickness is in general about 100 μm.
Then, in order that the aluminum base material may not be dissolved, a treatment with a constant voltage within a voltage range where the aluminum base material is not dissolved is effective, and for example, there is a method in which the treatment is started by means of a constant current treatment, and when the voltage reaches the “upper limit voltage” that is lower than a voltage at which the aluminum base material is dissolved, the treatment is switched to a constant voltage treatment with that “upper limit voltage”. However, when the treatment is switched to the constant voltage treatment by such a method, the current density is largely lowered, and the thickness is proportional to an accumulated quantity of electricity ((current density)×(treatment time)), namely a film formation rate ((thickness)/(time)) is proportional to the current density, and therefore, another problem that it takes a long time for the treatment, leading to deterioration of the productivity is caused.
In the light of the above, as a method for suppressing poor appearance or forming a thick film with a high speed, there are disclosed a method for forming an anode oxide film by applying an electrolyte to an article to be treated from a large number of electrolyte injection nozzles in an electrolyte bath; and the like (for example, Patent Documents 1 to 3). However, these technologies lead to an increase in costs by equipment investment, e.g. necessity of equipment for injection, etc.
Members on which an anode oxide film is formed may be required to have high hardness according to an application thereof, as seen in the foregoing semiconductor manufacturing apparatus equipment. However, it is the actual situation that techniques which have been proposed up to date cannot sufficiently treat those problems.
As a method for allowing an anode oxide film to have high hardness, for example, Patent Document 4 proposes a method for forming a high hardness anode oxide film using a sulfuric acid based electrolyte having an alcohol added thereto. However, this method involves such a problem that the control of a concentration change of the alcohol in the electrolyte by the anode oxidation treatment is complicated.
In addition, Patent Document 5 proposes a method for further forming an oxide sprayed film on a surface of a surface-treated member in which anode oxidation processing is applied to an aluminum alloy base material and discloses that the obtained film has high hardness. However, this method involves such problems that the treatment for forming the oxide sprayed film is very complicated; expensive equipment is required; and this method cannot be applied to a part of a complicated shape.
On the other hand, in an application as in semiconductor manufacturing equipment, from the viewpoint of suppressing a chemical reaction between the gas and the anode oxide film, there may be the case where a hydration treatment (commonly called sealing treatment) is applied to the anode oxide film. However, it is also known that in the case where the hydration treatment is conducted, the hardness of the anode oxide film is rather lowered (for example, Patent Document 6).